Sequence Stratigraphy and Platform Evolution of Lower–Middle Devonian Carbonates, Eastern Great Basin

Sequence stratigraphy and platform evolution of Lower–Middle Devonian carbonates, eastern Great Basin

Maya Elrick Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131

ABSTRACT thick) are composed of upward-shallowing INTRODUCTION successions of proximal turbidites or by Lower–Middle Devonian carbonates (270– platform-margin peloid shoal deposits; the Sequence stratigraphic concepts and ter- 400 m thick) of the eastern Great Basin absence of exposure features and meter- minology were originally defined from geo- were deposited along a low-energy, west- scale cycles within basin/slope sequence metric relationships of seismic reflectors in ward-thickening carbonate platform. Six re- boundary zones indicates that the combined siliciclastic systems (Vail et al., 1977). These gional facies representing peritidal, shallow rates of third- through fifth-order sea-level original concepts have been refined and in- subtidal, stromatoporoid biostrome, deep fall rates were less than tectonic subsidence tegrated with data from well logs, cores, and subtidal, slope, and basin environments rates. outcrops, and have incorporated the effects are recognized. Four third-order (Ϸ1.5–2.5 Sequence stratigraphic correlations be- of high-frequency sea-level oscillations 4 5 m.y. durations), transgressive-regressive se- tween contrasting facies belts of the ba- (10 –10 yr) to understand the internal ar- quences are identified across the platform- sin/slope (section NA) and the edge of the chitecture of depositional sequences (e.g., to-basin transition based on deepening and shallow platform (section TM) were inde- Wilgus et al., 1988; Goldhammer et al., shallowing patterns in regional facies, in- pendently verified with high-resolution co- 1990, 1993; Montan˜ez and Osleger, 1993). tensity and stratigraphic distribution of nodont and brachiopod biostratigraphy. These stratigraphic techniques have been subaerial exposure features, and stacking Correlation of sequences 1–4 with trans- successfully applied to outcrops across car- patterns of fourth- to fifth-order, upward- gressive-regressive sequences of similar age bonate platform-to-basin transitions where shallowing peritidal and subtidal cycles. in the western, midwestern, and eastern exposures are laterally continuous and per- Transgressive systems tracts along the United States, western Canada, and Europe mit tracing of stratal geometries and bound- basin/slope are characterized by upward- indicates they are eustatic in origin. aries physically or by photo mosaics (Sarg, deepening successions of proximal through 1988; Franseen et al., 1993; Sonnenfeld and Systems-tract scale correlations across distal turbidites overlain by fine-grained, Cross, 1993). In structurally complex regions the study area indicate that the platform hemipelagic deposits. Shallow-platform or where exposures are limited, such as the evolved from a homoclinal ramp to a dis- transgressive systems tracts are composed Great Basin, laterally continuous, unde- tally steepened ramp, then into a flat- of stacks of thicker-than-average peritidal formed outcrops are rare, and stratal geom- topped platform (sequences 1–2). An in- cycles overlain by subtidal cycles or noncyc- etries cannot be traced over long distances. cipiently drowned, intraplatform basin lic deep subtidal facies. Maximum flooding This paper discusses the sequence stratig- developed during sequence 3 as the result of zones along the shallow platform are com- raphy and evolution of a Lower–Middle De- third-order sea-level rise and differential posed of stacked peritidal cycles dominated vonian carbonate platform-to-basin system sediment accumulation rates between the by subtidal facies, noncyclic deep subtidal that developed in a passive-margin setting in facies, or distinct deeper subtidal units platform margin and intraplatform basin. the eastern Great Basin of the western within successions of restricted shallow During deposition of highstand systems United States (Fig. 1). The Lower–Middle subtidal or peritidal facies. Highstand sys- tract 3, progradation infilled the intraplat- Devonian deposits are present in block- tems tracts along the basin/slope are com- form basin, resulting in a flat-topped plat- faulted mountain ranges characteristic of posed of hemipelagic deposits overlain by form. A distally steepened ramp developed the Basin and Range Province; conse- distal through proximal turbidites. High- during transgressive systems tract/maxi- quently, tracing of stratal geometries and stand systems tracts along the shallow plat- mum flooding zone 4 and evolved into a flat- critical horizons is not possible between iso- form are characterized by upward-shallow- topped platform during highstand systems lated mountain ranges. In addition, biostrat- ing succession of cyclic peritidal through tract 4 deposition. The four sequences stack igraphically diagnostic fossils are present shallow subtidal facies. in an aggradational to slightly prograda- only in the deeper-water deposits, thus cor- Sequence boundary zones (2–16 m thick) tional pattern (‘‘keep-up’’ style sedimenta- relations based on high-resolution bio- along the shallow platform are composed of tion) and are bound by sequence boundary stratigraphy are limited. The main objec- exposure-capped peritidal and subtidal cy- zones rather than unconformities, suggest- tives of this paper are (1) to illustrate how cles that exhibit upsection increases in the ing that greenhouse climate modes and sec- one-dimensional stratal stacking patterns at proportion of tidal-flat subfacies and in- ond-order accommodation gains related to individual stratigraphic sections are used to creases in the intensity of cycle-capping sub- the lower portion of the second-order correlate depositional sequences across a aerial exposure features. Sequence bound- Kaskaskia sequence controlled sequence- full platform-to-basin transition; (2) to in- ary zones along the basin/slope (6–20 m scale stacking patterns. terpret platform evolution at the systems-

GSA Bulletin; April 1996; v. 108; no. 4; p. 392–416; 12 ﬁgures; 1 table.

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ably below Middle–Upper Devonian platform carbonates (Guilmette Formation and coeval deposits) (Fig. 2). These Devonian deposits form the top of a 4- to 7.5-km- thick succession of uppermost Precambrian through Devonian passive-margin carbonates and siliciclastics (Stewart and Poole, 1974; Bond and Kominz, 1984). During the latest Devonian–Early Mississippian, eastward-directed thrusting related to the Ant- ler orogeny juxtaposed lower–middle Pa- leozoic deep-water deposits over coeval shallow-water deposits along the Roberts Mountain thrust (Fig. 1; Roberts et al., 1958; Johnson and Pendergast, 1981; Speed and Sleep, 1982). Eight stratigraphic sections were measured along the platform-to-basin transition in Nevada (Fig. 1); six of the sections are autochthonous with respect to the Antler or- ogenic thrust belt, whereas the two more basinal sections were transported eastward by Antler-related and younger compressional events.

REGIONAL FACIES AND DEPOSITIONAL INTERPRETATIONS

Figure 1. Map of study area with location of measured sections and line of cross section Six regional facies are recognized across shown in Figure 3. Regional extent of intraplatform basin modiﬁed from Johnson et al. the platform-to-basin transition; in order of Sheep Pass, southern increasing water depths these facies are ؍ Schell Creek Range; SP ؍ Pahranagat Range; SC ؍ PR .(1989) peritidal, shallow subtidal, stromatoporoid ؍ Cherry Creek Range; NM ؍ Sunnyside, southern Egan Range; CC ؍ Egan Range; S -north- buildup, deep subtidal (including intraplat ؍ Table Mountain, Mahogany Hills; NA ؍ Newark Mountain, Diamond Range; TM ern Antelope Range. form basin), slope, and basinal; all but the basinal and slope facies have been dolo- tract scale, which alternated between a dis- alents (Fig. 2) were deposited along a west- mitized. Meter-scale, upward-shallowing cy- tally steepened ramp and a ﬂat-topped ward-thickening carbonate platform that ex- cles (or parasequences) are present in peri- platform; and (3) to interpret controls on tended along depositional strike from tidal through deep subtidal facies, but sequence-scale stacking patterns. southern Canada to southeastern Califor- cannot be discerned in basinal, slope, and In an earlier paper (Elrick, 1995), the nia, and Ϸ400 km across strike (Fig. 1; Os- intraplatform basin facies. As a result of ex- stratigraphy of the Lower–Middle Devonian mond, 1954; Johnson et al., 1989, 1991). tensive dolomitization and recrystallization, deposits was discussed and interpreted in Oceanic deposits lay to the west of the plat- meter-scale cycles are obscured within the the context of understanding the spatial and form, and the partially emergent Transcon- coarse crystalline dolomite subfacies. De- tailed descriptions of the regional facies and temporal distribution and origin of high-fre- tinental Arch lay to the east (Sandberg et al., meter-scale cycles are given in Elrick (1995) quency, upward-shallowing cycles or parase- 1982; Johnson et al., 1989, 1991). Through- quences. For brevity in this paper, basic and Table 1; brief environmental interpre- out the Early and Middle Devonian, the re- facies are summarized in table format (Ta- tations are given below. gional trend of the platform-to-basin slope ble 1) and only a brief discussion of inter- Four, large-scale (tens to hundreds of break was roughly north-south through cen- preted depositional environments is pro- meters thick), transgressive-regressive se- tral Nevada (Fig. 1; Johnson and Murphy, vided to facilitate sequence stratigraphic quences (sequences 1–4) are recognized interpretations. The reader is referred to El- 1984; Johnson et al., 1989). This relatively and correlated across the platform-to-basin rick (1995) for detailed discussions of facies, stable slope-break position had existed since transition using deepening and shallowing subfacies, meter-scale cycles, and cycle-gen- the Late Silurian (Llandovery) when fault- patterns in regional facies, the intensity and erating mechanisms. induced(?) downdropping caused the slope stratigraphic distribution of subaerial expo- break to backstep eastward Ϸ65 km (John- sure features, and changes in meter-scale, GEOLOGIC SETTING son and Potter, 1975). cycle stacking patterns. Middle Devonian Middle Devonian deposits lie discon- time scales of Odin et al. (1982), Palmer The uppermost Lower–Middle Devonian formably above Lower Devonian platform (1983), and Harland et al. (1982, 1989) in- Simonson Dolomite and stratigraphic equiv- dolomites (Sevy Dolomite) and conform- dicate the average duration of the sequences

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is Ϸ1.5–2.5 m.y.; as such, they are third-or- Nevada (CC, NM, and TM) (Figs. 1 and 3). restored. Consequently, the inner platform der in scale (Goldhammer et al., 1993). The term shallow ramp or platform refers portion of the cross section was constructed In this paper, the term platform is used in collectively to inner through outer ramp/ using present (or actual) distances. Dis- a general sense for a thick succession of platform sections because, at these loca- tances between dip-oriented sections CC, mostly shallow-water carbonates and does tions, the deposits are composed dominantly NM, TM, and NA were restored using data not imply any specific morphology (e.g., of shallow subtidal and peritidal facies. Ba- from Levy and Christie-Blick (1989) and Wilson, 1975; Read, 1985). The term ramp sinal and slope facies occur only at section Dobbs et al. (1993). refers to a gently dipping surface (Ͻ1Њ NA; consequently this section is referred to slope) where shallow-water carbonates pass as ‘‘basinal.’’ Peritidal Facies gradually into deeper-water carbonates When interpreting the regional cross sec- (Ahr, 1973; Read, 1985). As a result of long- tion shown in Figure 3, it is important to Peritidal facies include thin and thick term changes in accommodation space, the realize that inner platform sections are ori- laminite beds, laminated-wavy beds, and Middle Devonian carbonate platform ented parallel to depositional strike, where- monomict and polymict breccias (Table 1). changed from a ramp to flat-topped plat- as basinal and outer platform sections are These subfacies are arranged into upward- form, and the position of the shoreline dip-oriented sections. Because the magni- shallowing, meter-scale peritidal (capped by migrated; consequently, to simplify discus- tude and direction of Tertiary Basin and tidal-flat subfacies) and, less commonly, sub- sions, the terms ‘‘inner’’ and ‘‘outer’’ ramp/ Range extension and Mesozoic contraction tidal cycles (capped by subtidal subfacies). platform refer to present geographic posi- are similar between sections along the inner Thin to thick laminites subfacies are tions. The inner ramp/platform includes platform (sections PR, SC, SP, S, and CC; present across the shallow platform and stratigraphic sections in eastern Nevada Levy and Christie-Blick, 1989), the relative form caps (0.1–2.8 m thick) to peritidal cy- (sections PR, SC, S, and SP), and the outer distance between these sections does not cles and thin, transgressive bases (Ͻ0.1 m ramp/platform refers to sections in central change appreciably when palinspastically thick) to some cycles. Thin laminites repre-

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sent full aggradation of the seafloor to high- cipient pedogenesis, and alveolar structures thick) represent solution-collapse breccias intertidal and supratidal water depths (Har- (Elrick, 1995). (e.g., Kahle, 1988; Knight and James, 1991) die and Shinn, 1986). Thick laminites were Laminated-wavy beds (0.2–3.3 m thick) that developed during high-frequency (104– deposited in lower intertidal to restricted, are present at the base or in the middle part 105 yr) sea-level falls related to meter-scale shallowest subtidal environments as indi- of peritidal cycles across the shallow plat- cycle development (Elrick, 1995). No evi- cated by the conformable stratigraphic po- form. Ripple cross-laminae attest to traction dence of former evaporite minerals is ob- sition below thin laminites, rare skeletal and transport and graded planar laminae indi- served, thus it is likely that more soluble trace fossils, and rare interbedding with cate deposition from waning-flow, tidal, or phases such as aragonite and/or high-Mg thrombolites or LLH stromatolites. The storm currents. Restricted, shallow subtidal calcite were preferentially leached. The va- abundance of planar laminae indicates dep- conditions are indicated by the paucity of riety of clast compositions indicates that at osition from suspension settling and traction skeletal and trace fossils, the stratigraphic least several subfacies were involved in so- transport as subtidal sediment was washed position below tidal-flat subfacies, and by lution-collapse events. At section PR, se- onto the tidal flats during storms and high the lack of subaerial exposure features. quence 4 is composed of an Ϸ60-m-thick tides (i.e., mechanical sedimentation domi- Shallow subtidal environments have been polymict breccia unit (Fig. 3). The origin of nated algal binding). Both thin and thick interpreted for similar facies (‘‘ribbon this thick breccia unit is well understood; laminites display abundant evidence of sub- rocks’’) by Demicco (1983), Osleger and however, it is not likely related to high-fre- aerial exposure including sediment-filled Read (1991), and Cowan and James (1993). quency sea-level fluctuations, rather to events dissolution cavities, horizontal and vertical Monomict and polymict breccias are postdating Simonson Dolomite deposition. desiccation cracks, rubble and solution-col- present at the tops of some peritidal cycles Monomict rubble breccias represent the lapse breccias, microkarst erosion surfaces, along the inner and outer ramp. Cycle-cap- initial breakdown of bedrock and protosoil textural homogenization resulting from in- ping polymict breccia beds (0.08–4.0 m formation during periods of subaerial expo-

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ramp of sequence 1 (sections NA and TM). The abundance of open-marine fossil types (crinoids and brachiopods) and stratigraphic relationships with adjacent facies indicate deposition in normal marine, shallow subtidal environments.

Stromatoporoid Buildup Facies

Stromatoporoid floatstone/boundstone (0.3–4.0 m thick) caps subtidal cycles along the outer and inner platform, and isolated stromatoporoid heads occur locally in deep subtidal (intraplatform basin facies) and peloid-ooid packstone/grainstone subfacies. The diversity of skeletal material in the matrix between stromatoporoid heads (brachiopods, corals, gastropods, rare bryozoans, and crinoids) and the stratigraphic Figure 2. Chronostratigraphic and biostratigraphic chart for Lower and Middle Devo- association with deep subtidal facies indi- nian formations from shallow-water inner-platform (eastern Nevada) to deeper-water out- cate deposition in deep subtidal, open- er-platform and basin (central Nevada). Vertical lined pattern indicates unconformity. marine environments. The relatively fine- Modified from Kendall et al. (1983), Johnson et al. (1989), and Hurtubise (1989). grained texture of the matrix and the lack of current- or wave-generated features suggest the buildups did not act as wave-resistant sure (e.g., Meyers, 1988; Knight and James, position below tidal-flat subfacies, and anal- barriers or reefs; rather the buildups formed 1991). Fitted fabrics and host-attached ogies with modern oolitic and peloid grain- low-relief biostromes. clasts represent in situ fragmentation and stone deposits. Peloidal-ooid beds directly dissolution of lithified bedrock by down- overlain by tidal-flat deposits represent Deep Subtidal Facies (Including ward-flowing, undersaturated fluids. fringing shoals, whereas thick, noncyclic in- Intraplatform Basin Facies) tervals represent amalgamated shoals that Shallow Subtidal Facies developed along the outer platform. Deep subtidal facies are composed of bio- Amphipora dolowackestone/grainstone turbated skeletal mudstone/wackestone and Shallow subtidal facies include burrow- subfacies (0.7–3.0 m thick) is most common are present mainly as thick, noncyclic inter- mottled mudstone/wackestone, peloid-ooid along the outer platform and forms the base vals (12–47 m thick) along the inner and packstone/grainstone, Amphipora wacke- or the middle of peritidal cycles. The depos- outer platform of sequence 3, and at the stone/grainstone, coarse crystalline dolo- its are defined by the high abundance of base of sequence 1 (section NA; Fig. 3). In mite, and crinoidal packstone/grainstone Amphipora stromatoporoids. The presence particular, these facies form the base of subfacies (Table 1). They are arranged into of unabraded beds containing only Am- subtidal cycles developed along the inner peritidal and subtidal cycles. phipora fossils and the conformable associ- platform. The fine-grained texture, lack of Burrow-mottled dolomudstone/wacke- ation with overlying tidal-flat subfacies indi- wave- or current-generated sedimentary stone (0.3- to 5.5-m-thick intervals) is com- cate deposition in relatively low-energy, structures, and the presence of typical, mon along the shallow platform and forms restricted, shallow subtidal environments open-marine fauna indicate deposition be- the base or the middle of peritidal cycles, (Krebs, 1971; Wong and Oldershaw, 1980). low storm-wave base in normal marine en- and the tops of subtidal cycles. Deposition in Local patches of current-aligned fossils and vironments. These deep subtidal facies lie low-energy, moderately restricted, shallow grainstone layers suggest episodic, high-en- both seaward and landward of coeval shal- subtidal environments is indicated by the ergy conditions. low subtidal/peritidal facies (Fig. 3), indicat- abundance of Thalassinoides burrows, the Coarse crystalline dolomite (1- to 50-m- ing that a deeper-water intraplatform ba- paucity of skeletal fossils, the stratigraphic thick intervals) is present along the inner sin developed during sequence 3 (discussed position below tidal-flat subfacies, and the through outer ramp of sequence 1. Recrys- below). lack of subaerial exposure features. tallization and dolomitization have ob- Peloid-ooid packstone/grainstone is rare scured most primary features; however, the Slope Facies and forms the base (Ͻ4 m thick) of some presence of planar laminae, fenestral fab- peritidal cycles along the shallow platform rics, cross-bedding, stromatolites, and typi- Slope facies are present only in the ba- or forms thick, noncyclic intervals (Ͼ20 m cal shallow-water fossils, and stratigraphic sinal section NA and are not arranged into thick) along the outer platform (Fig. 3). The relationships with adjacent facies suggest meter-scale cycles. deposits represent current- or wave-agi- deposition in shallow-subtidal to tidal-flat Coarse crinoidal packstone/grainstone tated, shallow subtidal environments as in- environments. beds (Ͻ0.5 cm thick) are present in the up- dicated by coarse grain size, degree of sort- Crinoidal packstone/grainstone (1- to 11- permost part of each sequence and are in- ing, sedimentary structures, stratigraphic m-thick intervals) is present along the outer terpreted as upper slope or proximal turbid-

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Figure 3. Palinspastically restored cross section of the platform-to-basin transition. Location of measured sections is shown in Fig- ure 1. Note that distances shown in Figure 1 differ from the cross section because of palinspastic restoration using data of Levy and Christie-Blick (1989) and Dobbs et al. (1993). Note also that inner platform sections lie parallel to depositional strike, and outer platform and basinal sections are dip oriented. Stratigraphic datum of cross section is the maximum ﬂooding zone of sequence 3. Because a single datum within sequence 3 was used to ‘‘hang’’ the entire succession, overlying and underlying sequences are slightly distorted. Only 5 of the 11 formation and/or member boundaries coincide with sequence boundaries.

ites derived from outer platform and/or more distal turbidites deposited along the of slope and basinal rock types, and individ- upper-slope crinoid thickets. lower slope and toe of slope as indicated by ual beds commonly grade laterally into Laminated pellet-peloid packstone/grain- the ﬁne-grained texture and intercalation folded, slumped, or undeformed basin/slope stone lenses (Ͻ0.4 m thick, less than a few with basinal facies. facies indicating only minor downslope meters wide) are present in the basal and Lime-clast conglomerates (0.3- to 5.0-m- transport. This subfacies is interpreted to middle parts of sequence 2 and represent thick beds) contain tabular clasts composed represent debris-ﬂow beds deposited along

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the slope and toe of slope (e.g., Cook and cracks, sediment- and spar-filled dissolution cause it requires a period of nondeposition Taylor, 1977). cavities, rubble and solution-collapse brec- combined with gradual basin subsidence to cias, and incipient pedogenic features). cause transgression and relative deepening, Basinal Facies Nearly half of the peritidal cycles have a and the presence of abundant transgressive- basal transgressive unit (upward-deepening prone cycles indicates deposition during Basinal facies are present only at section facies trends) overlain by a typical regressive transgressions. Episodic subsidence (fault- NA. unit (upward-shallowing facies trends); induced downdropping; Cisne, 1986) is also Limestone-marl rhythmites (5- to 80-m- these are termed transgressive-prone cycles. precluded as a cycle-generating model be- thick intervals) were deposited below storm- Subtidal cycles are limited to the basal cause of the abundance of transgressive- wave base in poorly oxygenated basinal en- and middle portions of sequences and are prone cycles, which indicate gradual deep- vironments. Individual, submillimeter-thick, laterally equivalent to updip peritidal cycles. ening from tidal flat to subtidal water depths graded laminae within limestone and marl Two types of subtidal cycles are observed. rather than abrupt deepening, which would layers represent suspension deposition from Exposed subtidal cycles are composed of an be expected from fault-induced subsidence. either single storm events or distal turbidity upward-shallowing succession of shallow The preservation of upward-deepening fa- currents with the lime mud and pelleted ma- subtidal subfacies, and cycle caps display ev- cies trends at cycle bases also indicates that terial being derived from the adjacent shal- idence of subaerial exposure (sediment- and sediment redistribution by waves or currents low platform region. The rhythmic alter- spar-filled dissolution cavities and breccia- during transgressions was minor (low-ener- ation of limestone and marl layers likely tion). Submerged subtidal cycles are charac- gy transgressions). reflects climatically controlled changes in terized by deep subtidal facies gradationally (1) fluvial and/or eolian influx of terrigenous overlain by shallow subtidal subfacies; these DEPOSITIONAL SEQUENCES AND material into the marine environment; (2) subtidal cycle caps show no evidence of sub- SYSTEMS TRACTS storm tract location, which controlled the aerial exposure. Both subtidal cycle types in- transport of carbonate material derived dicate incomplete infilling of accommoda- Methods used to define depositional se- from the shallow platform; or (3) turbidity tion space, before sea-level fall in the case of quences in the Middle Devonian carbonate current generation, which controlled the exposed subtidal cycles, and before the suc- deposits differ from those used to define transport of siliciclastic and carbonate ma- ceeding transgression for submerged cycles. seismic-scale sequences, because onlapping terial to the basin (Elrick, 1993; Elrick and The average calculated duration of peritidal and downlapping stratal geometries are dif- Hinnov, in press). and subtidal cycles using the Odin et al. ficult to recognize along low paleoslopes Platy argillaceous wackestone (0.2- to 30- (1982), Palmer (1983), and Harland et al. and where stratigraphic sections are widely m-thick intervals) represents deposition be- (1989) time scales is between Ϸ50 and 130 separated between isolated Basin and low storm-wave base; the presence of small k.y.; as such, they are fourth- or fifth-order Range fault blocks. Instead, the Middle De- burrows and sparse skeletal material indi- in scale (Goldhammer et al., 1993). vonian sequences are identified from verti- cates an increase in bottom-water oxygen The mechanism that best explains the cal and lateral changes in regional facies, levels relative to the limestone-marl rhyth- abundance of cycle-capping exposure fea- from changes in meter-scale cycle stacking mites. The graded laminae are interpreted tures, transgressive-prone peritidal cycles, patterns (systematic vertical changes in cycle as the result of distal storm or distal turbid- and subtidal cycles correlative with updip subfacies and cycle thickness), and from the ite deposition. peritidal cycles is high-frequency (104–105 stratigraphic distribution and intensity of cy- yr) sea-level fluctuations (see Elrick, 1995, cle-capping subaerial exposure features. METER-SCALE for detailed discussion). Exposure-capped Sequences were initially identified from ver- UPWARD-SHALLOWING CYCLES peritidal and subtidal cycles are interpreted tical facies changes and cycle stacking pat- as the result of high-frequency sea level fall- terns in individual stratigraphic sections, Peritidal through deep subtidal facies are ing below the platform surface for one, or then similar patterns were correlated be- arranged into meter-scale upward-shallow- more likely, several sea-level oscillations tween adjacent sections to identify lateral or ing peritidal cycles (capped by tidal-flat (‘‘missed beats’’; Goldhammer et al., 1990). two-dimensional facies relationships of ret- laminites) and subtidal cycles (capped by Transgressive-prone cycles indicate that rogradation, aggradation, and progradation. shallow subtidal subfacies). Detailed de- subtidal sediment was being produced and The bed-by-bed resolution available from scriptions and interpretations of the cycles deposited during transgressions; that is, sed- this outcrop study reveals that the changes are given in Elrick (1995) and are briefly imentation lag times were minimal during between retrogradational and aggradation- outlined below to aid in illustrating how cy- initial flooding of the platform. Subtidal cy- al/progradation facies patterns (maximum cle stacking patterns and intracycle facies cles correlative with updip peritidal cycles flooding surface) and progradational to ret- composition are utilized to identify and cor- are interpreted to reflect incomplete shal- rogradational patterns (sequence bounda- relate sequences, systems tracts, and se- lowing and infilling of accommodation space ries) are gradational over meters to tens of quence boundaries. before the succeeding high-frequency rise in meters and are composed of stacked meter- Peritidal cycles (90% of the observed cy- sea level. Noncyclic intervals in the in- scale cycles along the shallow platform and cles) are present within each of the four traplatform basin, slope, and basin are in- noncyclic intervals along the basin/slope. shallow platform sequences. They are com- terpreted as the result of the seafloor lying These gradational or transitional maximum posed of shallow subtidal subfacies grada- too deep to resolve high-frequency sea-level flooding zones and sequence boundary zones tionally overlain by tidal-flat facies. Approx- oscillations (subtidal missed beats). reflect the effects of repeated fourth- to imately 80% of the peritidal cycle caps show The autogenic tidal-flat progradation fifth-order sea-level fluctuations superposed evidence of subaerial exposure (desiccation model of Ginsburg (1971) is precluded be- upon third-order relative sea-level events

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(see discussions in Goldhammer et al., 1993, subaerial exposure is recognized. Instead, meter-scale cycles are difficult to discern; and Montan˜ez and Osleger, 1993). These sequences are bounded by stratigraphic in- however, large-scale (several tens of me- transitional zones contrast with boundaries tervals (meters to tens of meters thick) of ters), transgressive-regressive facies trends identified seismically, because typical seis- exposure-capped cycles. Within these inter- can be identified and correlated to patterns mic data has the spatial resolution of a few vals, cycle caps typically exhibit progressive observed in undolomitized deposits along tens of meters; consequently, boundaries upsection increases in the intensity of subaer- the ramp margin and basin/slope. The base between retrogradational and aggrada- ial exposure features (positive chronose- of sequence 1 is a widespread unconformity tional/progradational facies patterns will ap- quences of Wright, 1994). For example, that separates the Lower Devonian Sevy pear as surfaces rather than stratigraphic shallow platform sequence boundary zones Dolomite from the uppermost Lower–Mid- zones. are typically composed of three to six peri- dle Devonian Simonson Dolomite (Johnson In the following sections, the stratigraph- tidal cycles; the caps to all of the cycles ex- and Murphy, 1984); traced basinward ic, sedimentologic, and biostratigraphic cri- hibit textural homogenization (laminite re- (west), the unconformity becomes a con- teria used to identify and correlate systems crystallization of Elrick, 1995) and alveolar formable succession in the Oxyoke Canyon tracts and their boundaries are described for structures related to incipient pedogenesis. and Sadler Ranch Formations (Kendall et shallow platform, platform margin, and ba- In addition to pedogenic features, the cycles al., 1983; Fig. 2). sin/slope regions for each of the sequences. toward the top of the succession also display At sections NA and TM, an upsection gra- Figure 4A (and Figures 6A, 8A, and 10A dissolution cavities filled with yellow-weath- dation from crinoidal packstone/grainstone below) illustrates lateral and vertical rela- ering argillaceous dolomite (cavities may ex- (shallow subtidal subfacies) to overlying dark- tionships of regional facies, meter-scale cy- tend 1 m below cycle cap). The cycles at the gray, skeletal wackestone (deep subtidal fa- cle thickness trends, and systems tracts for top of the succession may be capped by mo- cies) represents upward-deepening of the sequences 1–4. Detailed stratigraphic col- nomict rubble breccias that reflect extensive transgressive systems tract (sections NA and umns of portions of each sequence are pro- dissolution and fragmentation of the ex- TM, Fig. 4B). Along the rest of the shallow vided to illustrate the meter-scale details of posed surface. Each exposure-capped cycle ramp, the basal Lower Devonian sequence specific boundaries and systems tracts across is interpreted to represent a drop in fourth- boundary is directly overlain by a succession the platform-to-basin transect. to fifth-order sea level below the platform of sandy coarse crystalline to coarse crystal- surface and exposure for time periods on the line dolomite (coarse crystalline members of Sequence Boundaries order of 103–105 yr; successions of exposure- the Oxyoke Canyon Formation and Simon- capped cycles indicate that the shallow son Dolomite). This subfacies contains lo- The concept of sequences and sequence platform was affected by multiple episodes cal planar laminae, cross-stratification, and boundaries were originally defined from of subaerial exposure during a long-term stromatolitic horizons indicating deposition seismic-scale siliciclastic systems deposited accommodation minimum rather than a sin- in shallow subtidal to peritidal environments; on shelves with recognizable shelf-slope gle, long-lived exposure event. These strat- as such, they represent a transgression over breaks (Vail et al., 1977). Defining type 1 igraphic and diagenetic relationships indi- the unconformity. versus type 2 sequence boundaries based on cate that the rates of third-order sea-level At section NA, shallowing related to the the seaward extent and intensity of uncon- fall were less than tectonic subsidence rates overlying highstand systems tract is indi- formity development was, as a consequence, along the shallow platform, resulting in the cated by dark-gray, skeletal wackestone relatively straightforward. More recent se- developmentofthird-ordersequencebound- (deep subtidal facies) grading into lami- quence stratigraphic studies along carbon- ary zones rather than third-order sequence nated pellet-peloid pack-/grainstone (distal ate ramps that lack distinct slope breaks and bounding unconformities. Physical tracing turbidites), and capped by coarse-grained, in systems recording various orders of sea- of shallow platform sequence boundary graded crinoidal packstone/grainstone (prox- level oscillation highlight the fact that type 1 zones into downdip basin/slope regions is imal turbidites) (section NA, Fig. 4B). Shal- versus type 2 sequence boundaries are dif- not possible because of discontinuous out- lowing at section TM is represented by ficult to define (Burchette and Wright, 1992; crop exposures; however, basin/slope depos- dark-gray, skeletal wackestone (deep sub- Goldhammer et al., 1993; Montan˜ez and its show distinct shallowing-then-deepening tidal facies) gradationally overlain by me- Osleger, 1993). Instead, the boundaries facies changes over stratigraphic intervals of dium-bedded, crinoidal wackestone/pack- along these types of platforms are typically tens of meters, and these can be lithologi- stone (shallow subtidal subfacies). Updip conformable stratigraphic intervals repre- cally and biostratigraphically correlated to along the shallow ramp, the transition be- senting transitions between transgressive similar facies patterns observed along the tween retrogradational (transgressive sys- (retrogradational) and regressive (progra- shallow platform. tems tract) and aggradational/prograda- dational) depositional patterns (transitional tional (highstand systems tract) deposition sequence boundaries of Montan˜ez and Os- Depositional Sequence 1 cannot be discerned in the coarse crystalline leger, 1993). These transitional boundaries dolomite; however, late highstand systems are several meters to tens of meters thick Sequence 1 is 30–75 m thick and includes tract 1 is not as strongly recrystallized as the and are composed of stacked, high-fre- the Denay Limestone, upper Coils Creek underlying early highstand systems tract 1. quency cycles rather than discrete, laterally Limestone, middle and upper members of Consequently, a 6- to 28-m-thick interval of traceable surfaces. the Sadler Ranch Formation, the coarse stacked peritidal cycles is recognized (sec- Along the Middle Devonian platform, no crystalline members of the Oxyoke Canyon tions PR and CC, Fig. 4B). evidence of major penetrative karstification Formation, and Simonson Dolomite (Fig. 3). Sequence boundary zone 2 along the shal- or truncation related to prolonged (several Along the shallow ramp, deposits are so low ramp is identified as a 6- to 13-m-thick hundreds of thousands to millions of years) strongly dolomitized and recrystallized that interval of exposure-capped peritidal cycles.

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Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/108/4/392/3382390/i0016-7606-108-4-392.pdf by guest on 30 September 2021 Figure 4. (A) Cross section of sequence 1 illustrating systems tracts, boundaries, and changes in meter-scale cycle thickness. Symbol explanations shown in Figure 3. Dolomitization and recrystallization along the majority of the inner and outer ramp obscures meter-scale cycle recognition except during the late highstand systems tract 1. (B) Partial stratigraphic columns illustrating details utilized to interpret systems tracts and boundaries. Scale in meters.

400 Geological Society of America Bulletin, April 1996

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/108/4/392/3382390/i0016-7606-108-4-392.pdf by guest on 30 September 2021 The two to six cycles within this interval show upsection increases in the proportion of tidal-flat subfacies, and each cycle cap dis- plays evidence of prolonged subaerial exposure. For example, at the innermost ramp section PR, the caps of the upper four peritidal cycles defining sequence boundary zone 2 display sediment-filled dissolution cavities that extend 0.5–1.2 m below cycle tops; these features are absent in the underlying peritidal cycle caps (section PR, Fig. 4B). At each of the shallow ramp locations, sequence boundary zone 2 is directly overlain by a succession of peritidal cycles dominated by subtidal subfacies (section PR, Fig. 4B). At sections TM and NM, the stratigraphic interval corresponding to sequence boundary zone 2 is covered by Qua- ternary colluvium. At section NA (basin/slope), minimum accommodation related to sequence boundary zone 2 development is marked by the upsection transition from distal turbidites (laminated pellet-peloid pack-/grainstone) to proximal turbidites (coarse-grained, graded crinoidal packstone/grainstone). This Ϸ20-m-thick upward-shallowing succession is abruptly overlain by basinal facies intercalated with debris flows composed of slope-derived clasts (section NA, Fig. 4B). There is no evidence for subaerial exposure within this interval, implying that the combined rate of third-order and fourth- to fifth-order sea-level fall was less than tectonic subsidence along the basin/slope.

Platform Evolution During Sequence 1

The early transgressive systems tract of sequence 1 is characterized by peritidal to shallow subtidal facies passing seaward into shallow subtidal facies with no detectable break in slope; that is, a homoclinal ramp morphology (Fig. 5A). During maximum flooding (maximum flooding zone 1), a slight break in slope developed between sections NM and TM, evidenced by the deposition of shallow subtidal subfacies deposited at section NM, while sub-wave-base facies were deposited at the adjacent section TM (Fig. 5B). The lack of sediment gravity flow deposits within deep subtidal facies at sections TM and NA indicates that the gradient along this newly formed slope was not steep enough to generate allochthonous deposits. During deposition of highstand systems tract 1, the slope break migrated seaward to between sections TM and NA, and the slope gradient steepened enough to generate abundant distal to proximal turbidites; Figure 4. (Continued). that is, a distally steepened ramp (Fig. 5C).

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slope (section NA) is recorded by an abrupt vertical change from coarse crinoidal turbidites (underlying sequence boundary zone 2) to platy, argillaceous wackestone (basinal facies) intercalated with lime-clast conglomerates (slope-derived debris flow beds) (section NA, Fig. 4A). Continued deep-water conditions (late transgressive systems tract 2) is recorded by a thick, monotonous succession of limestone-marl rhythmites (basinal facies). Updip at section TM, a succession of peritidal cycles overlain by exposed subtidal cycles reflects an increase in accommodation space related to transgressive systems tract 2 development. Along the outer and inner platform, evidence for upward-deepening is more subtle than that displayed in downdip regions, but is indicated by successions of peritidal cycles that display upsection increases in the proportion of subtidal subfacies and an associated upsection increase in the abundance and diversity of skeletal fossils within the subtidal units (section CC, Fig. 4B); both characteristics suggest an increase in third-order accommodation space. The maximum flooding zone separating transgressive systems tract 2 from highstand systems tract 2 is not recognized along the basin/slope because of poor outcrop exposure. At section TM, facies representing maximum water depths occur within a single Figure 5. Schematic depositional profiles of ramp evolution during sequence 1. Symbols peritidal cycle whose base is composed of approximate location of section TM. (A) During early transgressive ؍ as in Figure 7; TM dark-gray, stromatactis-bearing dolowacke- systems tract 1, a homoclinal ramp morphology developed. (B) During maximum flooding stone with in-growth-position colonial cor- zone 1, a slight break in slope developed between sections NM and TM (incipient distally als (1.5-m-thick interval); in comparison, steepened ramp). (C) During deposition of highstand systems tract 1, the slope break underlying (transgressive systems tract) and prograded seaward to between sections TM and NA, and a distally steepened ramp mor- overlying (highstand systems tract) cycles phology developed. contain subtidal units composed of biotur- bated Amphipora wackestone (restricted To recapitulate, during sequence 1 the tems tract 1 resulted in the westward migra- shallow subtidal subfacies). Along the inner platform evolved from a homoclinal ramp tion of the slope break and a concurrent and outer platform, maximum deepening is during transgressive systems tract 1, to a steepening of the slope gradient. represented by a 7- to 20-m-thick succession ramp with a slight break in slope between of subtidal and peritidal cycles that are dom- sections NM and TM during the maximum Depositional Sequence 2 inated by shallow subtidal units (burrow- flooding zone (incipient distally steepened mottled wackestone); in contrast, underly- ramp), to a distally steepened ramp with the Sequence 2 is 95–180 m thick and includes ing and overlying cycles are composed slope break between section TM and NA part of lower Denay Limestone, Sentinel dominantly of tidal-flat subfacies (section during highstand systems tract 1. During the Mountain Dolomite, and the lower alternat- PR, Fig. 6B). Within maximum flooding late transgressive systems tract 1, when the ing member of the Simonson Dolomite zone 2, a thin marker bed (Ͻ0.5 m) com- rate of third-order accommodation gain was (Fig. 3). The sequence is characterized by posed of Stringocephalus brachiopod–bear- the greatest, relatively rapid shallow-water deepening and retrogradation of basinal, ing wackestone can be correlated between sedimentation rates along the shallow ramp slope, and subtidal through peritidal facies, four shallow platform stratigraphic sections kept pace with the accommodation gain, followed by shallowing and progradation of or Ϸ140 km along depositional strike while deeper subtidal accumulation rates shallow subtidal through peritidal facies (Fig. 6A); the presence of this marker bed lagged behind the increase in accommoda- (Fig. 6A). Isolated stromatoporoid build- attests to the isochronous nature of the max- tion. As a result, the gradient between shal- ups developed along the platform margin imum flooding zone. low and deeper water environments steep- throughout sequence development. Shallowing along the basin/slope (high- ened to generate a ramp with a slight break An upward-deepening succession (trans- stand systems tract 2) is subtle but is rep- in slope. Progradation during highstand sys- gressive systems tract 2) along the basin/ resented by thin-bedded limestone-marl

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rhythmites (basinal facies) grading upsec- phology evolved during late highstand sys- tion by peritidal, shallow subtidal, and slope tion into medium-bedded, nodular rhyth- tems tract 2, indicated by the presence of facies (Fig. 8A). mites interbedded with laminated peloid cyclic peritidal facies across the entire shal- Initial deepening marking transgressive packstone/grainstone (distal turbidites) (sec- low-platform region (Fig. 7B). A flat-topped systems tract 3 along the basin/slope is in- tion NA, Fig. 6B). Along the platform mar- morphology is also indicated by the fact that dicated by a 50-m-thick interval of lime- gin, shallowing is evidenced by a succession during the succeeding transgressive systems stone-marl rhythmites (basinal facies) im- of upward-thinning, exposed subtidal cycles tract 3, deepening is expressed by similar mediately overlying the underlying sequence reflecting a decrease in third-order accom- facies deposited over the entire region boundary zone 3 (section NA, Fig. 6B). modation space. Shallowing related to the (Fig. 3). Deepening along the platform margin (sec- highstand systems tract along the outer and The development of peritidal cycles tion TM) is recorded by a succession of inner platform is evidenced in individual across the entire shallow platform thicker-than-average subtidal and peritidal sections by successions of peritidal cycles throughout sequence 2 deposition indicates cycles, which are dominated by subtidal dominated by tidal-flat subfacies (sections that water depths in this region never ex- units reflecting the increase in accommoda- NM and SP, Fig. 6B), and regionally by the ceeded shallow subtidal water depths (Ͻ20 tion space (section TM, Fig. 6B). At sections progradation of a thick peritidal wedge over m), and the platform remained nearly ag- NM and CC, abrupt deepening is indicated shallow subtidal facies (Fig. 6A). graded to sea level throughout sequence de- by noncyclic intraplatform basin facies Sequence boundary zone 3, along the velopment (‘‘keep-up’’ style deposition of (Woodpecker Limestone) lying immediately shallow platform, is recognized as a 7- to Schlager, 1981). The flat-topped or ag- above peritidal cycles of the underlying se- 16-m-thick interval of peritidal cycles whose graded nature of the platform, particularly quence boundary zone 3 (section NM, caps display sediment-filled dissolution cav- during highstand systems tract 2, indicates Fig. 6B). At sections SP, S, and SC, a suc- ities, rubble and solution-collapse breccias, that sediment accumulation rates kept pace cession of submerged subtidal cycles grading and microkarst erosion surfaces. These ex- with accommodation space gains. As a con- up into noncyclic intraplatform basin facies posure-capped cycles are immediately over- sequence of the shallow platform remaining (brown cliff member) indicates an increase lain by noncyclic, intraplatform basin facies nearly aggraded to sea level throughout se- in third-order accommodation or transgres- or a succession of submerged subtidal cycles quence development, large portions of the sive systems tract 3 deposition (section SP, (sections NM and SP, Fig. 6B). Minimum shallow platform were subaerially exposed Fig. 6B). At the innermost platform section accommodation along the platform margin during fourth- to fifth-order sea-level low- (section PR), exposure-capped peritidal cy- is difficult to recognize from regional facies stands. Until additional accommodation cles of the underlying sequence boundary patterns alone; however, the highstand sys- space was generated by continued subsid- zone are abruptly overlain by stromatopo- tems tract succession of exposed subtidal ence or by succeeding higher amplitude roid buildup facies, indicating an increase in cycles is immediately overlain by an up- fourth- to fifth-order sea-level oscillations, accommodation space (section PR, Fig. 8B). ward-thickening succession of subtidal and the position of the shallow platform re- Distinct facies changes signaling maxi- peritidal cycles that are dominated by deep- mained above the effects of the smaller am- mum flooding along the basin/slope is not er-water subtidal subfacies (section TM, plitude oscillations, and the sedimentary observed because of poor exposure. Maxi- Fig. 6B). record of one or more high-frequency sea- mum water depths at section TM are re- Along the basin/slope, the third-order ac- level oscillations was not recorded (‘‘missed corded in a single peritidal cycle containing commodation minimum is subtle, but is re- beats’’; Elrick, 1995). a 5-m-thick unit of platy and bulbous stro- corded as an 8-m-thick interval of medium- matoporoid floatstone/boundstone (moder- bedded, nodular limestone-marl rhythmites ately deep subtidal facies) (section TM, (with sparse fossils) that are capped by a Depositional Sequence 3 Fig. 8B). Along the intraplatform basin, thin crinoidal packstone/grainstone bed. maximum water depths are represented by a This succession is abruptly overlain by a Sequence 3 is 90–130 m thick and in- 5- to 17-m-thick interval of thin-bedded, thick interval of thin-bedded limestone-marl cludes the Denay Limestone, Woodpecker noncyclic, deep subtidal facies that contain a rhythmites (basinal facies) of the overlying Limestone, basal Bay State Dolomite, and greater abundance of open-marine fossils transgressive systems tract 3 (section NA, the brown cliff and basal part of the lower (crinoids, whole brachiopods, tentaculitids, Fig. 6B). alternating members of the Simonson Do- and bryozoans) than surrounding intraplat- lomite (Fig. 3). The sequence is character- form basin facies (section SP, Fig. 8B). At Platform Evolution During Sequence 2 ized by abrupt and extensive deepening atop the most landward section PR, maximum the underlying sequence boundary zone 3 flooding zone 3 is similar to that developed Transgressive systems tract 2 inherited and by the formation of a deep subtidal, in- at section TM; maximum water depths occur the distally steepened ramp morphology of traplatform basin along the former flat- within a single subtidal cycle containing a the underlying sequence (Fig. 7A). A slight topped platform. During deepening, stro- 1.5-m-thick unit composed of platy stroma- seaward dip along the shallow ramp during matoporoid buildups developed along the toporoid boundstone; underlying and over- transgressive systems tract 2, maximum inner platform (as far landward as section lying cycles deepen only to shallow subtidal flooding zone 2, and early highstand systems PR), while deep subtidal facies of the in- water depths represented by Amphipora tract 2 is indicated by a thicker accumulation traplatform basin (Woodpecker Limestone packstone and burrow-mottled wackestone of shallow subtidal facies (composed of peri- and parts of the brown cliff member) were subfacies. tidal cycles dominated by shallow subtidal deposited as far landward as section S Shallowing related to highstand systems units) along the outer ramp than the inner (Fig. 8A). This deepening was followed by tract 3 is indicated along the basin/slope by ramp (Fig. 6A). A flat-topped platform mor- relatively abrupt shallowing and prograda- an upward-shallowing succession of thin-

Geological Society of America Bulletin, April 1996 403

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/108/4/392/3382390/i0016-7606-108-4-392.pdf by guest on 30 September 2021 Figure 6. (A) Cross section of sequence 2; symbols as in Figure 3. (B) Partial stratigraphic columns illustrating details utilized to interpret systems tracts and boundaries. Sym- bols as in Figure 4B. Scale in meters.

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Figure 6. (Continued).

bedded, limestone-marl rhythmites (basinal in third-order accommodation space. At in infilling and shallowing of much of the facies) grading into medium-bedded, nodu- sections CC and NM, intraplatform basin accommodation space and allowed cyclic lar rhythmites with intercalated lime-clast facies are overlain by a 4- to 25-m-thick in- peritidal facies to prograde across the conglomerates (upper slope-derived debris- terval of noncyclic, peloid-ooid packstone/ former intraplatform basin region. flow beds), and overlain by well-sorted pe- grainstone. On the landward (southeastern) The development of an intraplatform ba- loid grainstones (platform-margin shoals) side of the intraplatform basin, shallowing sin during transgressive systems tract 3 and (section NA, Fig. 8B). Along the platform related to highstand systems tract 3 is indi- maximum flooding zone 3 is interpreted margin (section TM), highstand systems cated by peritidal, shallow subtidal, and from the presence of noncyclic, deep sub- tract 3 is characterized by a succession of stromatoporoid buildup facies prograding tidal facies lying landward (or east) of cyclic upward-thinning peritidal cycles that exhibit seaward over intraplatform basin facies peritidal and shallow subtidal facies along a concurrent increase in the proportion of (sections S and SP, Fig. 8B). Progradation the platform margin (section TM, Fig. 8A). tidal-flat subfacies, which reflects a decrease from both sides on the basin margin resulted Regionally, these intraplatform basin facies

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tion cavities extending Ϸ0.3 m down from cycle tops, rubble breccias, and microkarst erosion surfaces (sections CC and S, Fig. 8B). Along the platform margin, sequence boundary zone 4 is represented by a 7-m-thick interval composed of a polymict breccia bed (1–4 m thick) overlain by three exposure- capped peritidal cycles. Along the basin/slope, sequence boundary zone 4 is represented by a 12-m-thick interval of peloid grainstones (platform- margin shoals) that are abruptly overlain by platy, argillaceous wackestone (basinal facies) of the overlying transgressive systems tract 4 (section NA, Fig. 8B). Progradation of platform-margin shoals over former basin/slope environments indicates that the slope break now lay west of the study area; prior to this, the slope break lay between sections TM and NA.

Platform Evolution During Sequence 3

A profound change in platform morphology occurs at the base of sequence 3. The relatively flat-topped platform of the underlying sequence deepens abruptly landward of section TM and substorm–wave-base water depths were attained across the majority of the study area (Fig. 9A). During deposition of transgressive systems tract 3 and maximum flooding zone 3, sediment accumulation rates did not keep pace with accommodation gains resulting in the development of an incipiently drowned intraplatform basin. It is not clear why accumulation rates at section TM were able to Figure 7. Schematic depositional profile of platform morphology during sequence 2. kept pace with accommodation gains while section TM. (A) A distally steepened ramp morphology developed during the late rates landward of this region lagged behind ؍ TM transgressive systems tract 1/maximum flooding zone 1 with shallow subtidal and stro- accommodation gains. matoporoid buildup facies deposited along the outer ramp. (B) Seaward progradation of The decrease in accommodation space cyclic peritidal facies during late highstand systems tract 2 generated a flat-topped plat- during highstand systems tract 3 resulted in form morphology with a slope break lying between sections TM and NA. The gradient along the infilling of the intraplatform basin from the slope was steep enough to generate minor sediment gravity flow deposits. both margins (Fig. 9B). Infilling of the intraplatform basin was complete by late highstand systems tract 3; subsequently, a relatively flat-topped platform morphology are present from southeastern California to ographically higher than the intraplatform developed landward of section TM (Fig. 9C). the Idaho-Nevada border and in an Ϸ20- to basin, it did not act as an energy barrier be- 100-km-wide facies belt (Fig. 1). Intraplat- cause platform margin deposits lack typical Depositional Sequence 4 form basin facies were deposited in deeper high-energy sedimentary structures/textures and more open-marine waters than coeval (i.e., cross-beds, winnowed grainstones, and Sequence 4 (55–70 m thick) includes the platform deposits as suggested by (1) the wave-resistant reef structures). Denay Limestone, the middle portion of the lack of coarse-grained textures or wave- or Sequence Boundary Zone 4. Along the Bay State Dolomite, and the upper portion current-reworked sedimentary structures, shallow platform, minimum third-order ac- of the upper alternating member (Fig. 3). (2) the lack of meter-scale cycles or subaer- commodation is recorded in an 8- to 12-m- Regional facies patterns are characterized ial exposure features, and (3) the presence thick zone of peritidal cycles whose caps by minor deepening and retrogradation of of typical open-marine fossils (crinoids, co- show abundant evidence for prolonged sub- basin/slope and shallow subtidal facies, fol- lonial corals, tentaculitids, and bryozoans). aerial exposure such as alveolar structures, lowed by basinward progradation of peri- While the platform margin was clearly top- desiccation cracks, sediment-filled dissolu- tidal through shallow subtidal facies

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(Fig. 10B). The most landward section (sec- 10–13 m of the highstand systems tract at grainstones, basinal facies, overlain by plat- tion PR) is composed wholly of polymict sections S and SC are composed of ooid form-margin grainstones), indicates that karst breccia obscuring facies trends. grainstone–based peritidal cycles (section S, third-order accommodation gains during Upward-deepening facies relationships Fig. 10B). transgressive systems tract 4/maximum related to transgressive systems tract 4 is Sequence Boundary Zone 5. Minimum ac- flooding zone 4 were great enough to estab- well developed along the basin/slope, while commodation along the shallow platform is lish subwave-base water depths atop former platform-margin and shallow platform re- defined by the vertical change from expo- grainstone shoals, and that the slope be- gions display only subtle changes in regional sure-capped peritidal cycles to an Ϸ15-m- tween the platform margin and basin during facies trends. Upward-deepening and retro- thick interval of noncyclic shallow subtidal transgressive systems tract 4/maximum gradation defining transgressive systems facies (sections S and TM, Fig. 10B). Along flooding zone 4 was gentle enough to permit tract 4 along the basin/slope is indicated by the inner platform, this shallow subtidal- the shoals to prograde seaward across the platform-margin grainstones abruptly over- dominated interval represents the basal Fox slope and infill the remaining topography. lain by a 5-m-thick interval of platy, argilla- Mountain member of the Simonson Dolo- These relationships indicate that the volume ceous wackestone (basinal facies) (section mite (Fig. 2). of sediment produced at section NA was NA, Fig. 8B). Maximum flooding presuma- At section NM, the upper7mofhigh- great enough to infill the underlying topog- bly occurs within this thin interval because it stand systems tract 4 is composed of light- raphy and permit progradation of the slope is immediately overlain by platform margin- beige, coarse crystalline peloidal dolomite; break. Slopes greater than a few degrees derived debris flow beds, then peloid grain- this is abruptly overlain by platy, argilla- would likely have restricted seaward progra- stone shoal deposits (highstand systems ceous wackestone (basinal facies) interbed- dation of autochthonous grainstones be- tract 4). Upward-deepening facies trends at ded with platform margin–derived debris cause of the greater water depths and gra- section TM are subtle; the stratigraphic in- flow beds (slope facies) (section NA, dients developed along the slope. terval above the underlying sequence Fig. 10B) and indicates a rapid increase in boundary zone 4 is composed of a succes- third-order accommodation space related to BIOSTRATIGRAPHIC CORRELATION sion of peritidal cycles that have a higher the overlying sequence. The upward-deep- OF SEQUENCES proportion of subtidal facies than the un- ening succession above sequence boundary derlying succession of peritidal cycles. Max- zone 5 represents the Taghanic onlap of Biostratigraphically diagnostic (at the imum water depths (maximum flooding Johnson (1970), a relatively long-lasting substage level) conodonts and brachiopods zone 4) at section TM are recorded in a sin- transgression recognized in the western are present only in basin/slope (section NA) gle, exposed subtidal cycle that contains a United States, New York, Belgium, and and some platform-margin deposits (section 0.5-m-thick brachiopod wackestone unit; Germany (Johnson et al., 1985). TM); intraplatform basin through peritidal subtidal units in underlying (transgressive facies, which compose the majority of the systems tract) and overlying (highstand sys- Platform Evolution During Sequence 4 sequences, contain fossils that can only con- tems tract) cycles are composed of shallow strain the deposits to a Middle–Late Devo- and/or restricted subtidal deposits (Am- During transgressive systems tract 4 and nian age (i.e., Stringocephalus brachiopods). phipora wackestone or wavy-laminated maximum flooding zone 4, a distally steep- The high-resolution biostratigraphic control beds) (section TM, Fig. 10B). Along the ened ramp developed with the break in available in the basin/slope section and the shallow platform, increases in third-order slope between sections TM and NA adjacent platform-margin section permits accommodation (transgressive systems tract (Fig. 11A). A slight westward-dipping gra- the correlation between contrasting facies 4) are indicated by a succession of peritidal dient along the outer ramp is indicated by belts; that is, noncyclic subwave-base depos- cycles that are dominated by subtidal units thicker successions of shallow subtidal facies its (section NA) can be correlated biostrat- at sections SC, SP, and CC. Maximum flood- at sections TM and NM than updip section igraphically with cyclic, shallow subtidal and ing zone 4 is best developed along the outer CC; in a landward direction, the transgres- peritidal deposits (section TM). Correlation platform where subtidal units within periti- sive systems tract 4/maximum flooding zone from the platform margin landward across dal cycles contain more open-marine fossils 4 is composed of cyclic peritidal facies, in- the rest of the shallow platform is based on (Stringocephalus brachiopods, colonial cor- dicating that the inner part of the platform distinct vertical and lateral facies patterns, als, and in-growth-position stromato- was relatively flat topped. During deposition cycle stacking patterns, and subaerial expo- poroids) than underlying and overlying peri- of highstand systems tract 4, cyclic peritidal sure features described above for individual tidal cycles (subtidal units composed of facies prograded seaward across the rest of systems tracts. Amphipora wackestone and wavy-laminated the platform, and a flat-topped morphology The transgressive systems tracts and high- subtidal subfacies) (section CC, Fig. 10B). was reestablished across the majority of the stand systems tracts of sequences 1–4 along Distinct shallowing and highstand systems study area. Grainstone shoals developed be- the basin/slope have been biostratigraph- tract development along the basin/slope are tween sections TM and NA and prograded ically dated using conodonts and/or brachio- indicated by slope facies overlain by a 60- westward over underlying basin/slope facies pods (Johnson et al., 1980, 1989, in press); m-thick interval of peloid grainstones with during highstand systems tract 4 (Fig. 11B); transgressive systems tracts have been dated up to 10% quartz sand (section NA, consequently, the location of the platform in sequences 1 and 4 at section TM (Fig. 3) Fig. 10B). Along the shallow platform (in- margin and the gradient along the slope can- (Johnson et al., 1980). The maximum flood- cluding section TM), highstand systems not be discerned within the study area. ing zone 1 at section NA is in the serotinus tract 4 is composed of stacked peritidal cy- The vertical succession of facies at section conodont Zone (late Emsian; Fig. 3; John- cles that are dominated by tidal-flat subfa- NA from highstand systems tract 3 to high- son et al., 1980). At section TM, the maxi- cies (section TM, Fig. 10B). The upper stand systems tract 4 (platform-margin mum flooding zone 1 is represented by the

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Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/108/4/392/3382390/i0016-7606-108-4-392.pdf by guest on 30 September 2021 Figure 8. (A) Cross section of sequence 3; symbols as in Figure 3. Note absence of cycles within the broad intraplatform basin. (B) Partial stratigraphic columns illustrating details utilized to interpret systems tracts and boundaries. Symbols as in Figure 4A. Scale in meters.

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Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/108/4/392/3382390/i0016-7606-108-4-392.pdf by guest on 30 September 2021 basal Sadler Ranch Formation, which is in the serotinus Zone (Fig. 3; Kendall et al., 1983). The Sadler Ranch Formation interfingers with the Oxyoke Canyon Formation (Fig. 3; Kendall et al., 1983) which, in turn, interfingers landward (east) with the coarse crystalline member of the Simonson Dolo- mite (Johnson et al., 1985, 1989). The last two units directly overlie the regional Lower Devonian unconformity and show similar transgressive and regressive facies patterns as the biostratigraphically controlled portion of sequence 1 at section TM. The transgressive systems tract 4/maximum flooding zone 4 along the basin/slope occurs within the ensensis conodont Zone (Johnson et al., 1980, in press). At section TM, the maximum flooding zone 4 occurs within western United States Faunal Inter- val 20 (containing Geranocephalus truncatus brachiopods; Johnson, 1978); Faunal Inter- val 20 immediately overlies the ensensis Zone, indicating that initial deepening (early transgressive systems tract 4) is within the ensensis Zone. The biostratigraphic and sequence stratigraphic correlations of sequences 1 and 4 bracket the ages of intervening sequences 2 and 3. Consequently, conodont dates from the transgressive systems tract and/or maximum flooding zone of sequences 2 and 3 along the basin/slope indicate that sequence 2 is in the middle costatus Zone, and sequence 3 is in the middle kockelianus Zone (Fig. 3; Johnson et al., in press). Each of the biostratigraphically dated systems tracts at section NA has been recognized and dated in Idaho and Montana, the

Figure 8. (Continued).

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midwestern and eastern United States, western Canada, and Europe (Johnson et al., 1985, in press). These Early–Middle Devo- nian deepening events have been utilized along with other Devonian events by John- son et al. (1985, 1989) to generate a third- order sea-level curve for the Devonian (Fig. 12). The third-order sea-level ﬂuctua- tions of the latest Early–Middle Devonian, termed T-R cycles Ic, Id, Ie, and If by John- son et al. (1985), correlate biostratigraphically with sequences 1, 2, 3, and 4, respectively; this intrabasinal and interbasinal correlation implies a eustatic control on the uppermost Lower–Middle Devonian sequences recognized in this study.

DEPOSITIONAL CONDITIONS DURING PLATFORM DEVELOPMENT

Several lines of evidence indicate that low-energy conditions prevailed across the Middle Devonian platform. First, typical high-energy deposits (peloid-ooid grainstones) are restricted to specific locations, were deposited during limited time intervals, and compose Ͻ20% of the Lower– Middle Devonian succession. Second, stromatoporoid buildups are associated with fine-grained rather than coarse-grained deposits, indicating that they were not wave- resistant structures that modified surrounding depositional environments. Third, Ͼ40% of the fourth- to fifth-order cycles preserve upward-deepening facies trends at their bases (transgressive-prone cycles), implying that high-frequency transgressions Figure 9. Schematic depositional profiles of platform morphology during sequence 3. section TM. (A) During deposition of transgressive systems ؍ were low enough in energy that transgres- Symbols as in Figure 7; TM sive deposits were not redistributed offshore tract 3/maximum flooding zone 3, a deep subtidal intraplatform basin developed along by wave or tidal processes. Fourth, trans- former shallow platform. (B) During the early highstand systems tract 3, ooid shoal de- gressive-prone cycles lie directly above del- posits prograded landward (east) and cyclic peritidal facies prograded seaward to infill the icate, cycle-capping microkarst features that intraplatform basin. (C) The loss of third-order accommodation during late highstand show no evidence of abrasion or reworking systems tract 3 resulted in the progradation of cyclic peritidal and platform-margin facies by shoreline currents and waves (see Elrick, across the intraplatform basin and basin/slope deposits, respectively. Because of late high- 1995, for further discussion). stand systems tract 3 progradation, the location of the slope break and the gradient along The interpreted low-energy conditions the slope cannot be observed within the study area. may be explained by prevailing paleogeo- graphic conditions. Middle Devonian paleo- Klein, 1983). In addition, during Devonian SEQUENCE-SCALE STACKING geographic reconstructions of Scotese and time, an eastward-migrating volcanic island PATTERNS McKerrow (1990), Witzke (1990), and Van arc presumably lay some distance seaward der Voo (1993, p. 263) place the western of western North America and collided with Fourth- to fifth-order or high-frequency United States between paleolatitude 10ЊS the continent during the latest Devonian– cycle stacking patterns (graphically illus- and 20ЊS, with the paleoshoreline trending Early Mississippian Antler orogeny (John- trated using Fischer plots) have been used to roughly northeast-southwest (present coor- son and Pendergast, 1981; Speed and Sleep, estimate changes in third-order accommo- dinates) (Fig. 1). Although these low paleo- 1982; Burchfiel and Royden, 1991). Prior to dation space (Read and Goldhammer, latitudes were likely influenced by hurri- collision, the island arc likely acted as a bar- 1988). The premise for the use of cycle canes, it is unlikely that westward- or rier to open-ocean storm waves and winds, stacking patterns is that the combined ef- southwestward-moving, Southern Hemi- creating relatively low-energy conditions fects of third-order through fifth-order sea- sphere hurricanes would have affected the along the western continental margin of level rises generate successions of thicker- western sides of continents (Marsaglia and North America. than-average cycles that exhibit evidence of

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dominantly submergent conditions. The loss tion space is also indicated by the develop- which of the remaining three mechanisms, of accommodation space during relative ment of a sequence boundary zone (indicat- in addition to third-order eustatic sea-level third-order sea-level fall and superposed ing third-order sea-level fall was less than rise, generated the incipient drowning event; high-frequency sea-level fall generates suc- tectonic subsidence across the study area) however, differential sedimentation rates cessions of thinner-than-average cycles that rather than a single, regional unconformity. combined with sea-level rise have been in- are dominated by peritidal facies and exhibit During sequence 2, the generation of a voked for other Paleozoic intrashelf basins evidence of prolonged subaerial exposure. thick wedge of shallow subtidal facies during in North America (e.g., Markello and Read, Stacking patterns of cycles composed wholly late transgressive systems tract 2/early high- 1982; Read, 1985). of deeper subtidal facies should be used with stand systems tract 2, the development of an Transgressive systems tract 3 and early caution, because the thickness of these cy- aggradational platform-margin geometry, highstand systems tract 3 along the basin/ cles is controlled by sediment accumulation and the development of a sequence bound- slope are very similar in lithology and facies rates rather than sea-level–controlled ac- ary zone reflect continuing increases in sec- patterns to transgressive systems tract 2 and commodation space. Consequently, succes- ond-order accommodation space. The ag- early highstand systems tract 2; this is true sions of thinner-than-average, deep sub- gradational facies patterns along the basin/ despite the differences in shallow platform tidal–dominated cycles reflect slower slope also support this interpretation morphologies between the two sequences. sedimentation rates in deeper water envi- (Fig. 12). This suggests that depositional processes ronments rather than a relative fall in third- The formation of an incipiently drowned, along the shallow platform were decoupled order sea level. intraplatform basin during transgressive sys- with the basin/slope. This decoupling is also Stacking patterns of third-order se- tems tract 3 and maximum flooding zone 3 indicated by the fact that in each of the ba- quences can be used in a similar manner to suggests a rapid and/or large magnitude in- sin/slope sequences, there is no evidence of estimate changes in second-order (long- crease in the rate of second-order accom- nondeposition or slow deposition (con- term) accommodation space. Intrasequence modation gain along the shallow platform, densed intervals, hardgrounds, or other fea- facies distributions, facies-tract geometries, hence the interpreted inflection in the sec- tures) that would correspond to times of platform morphology changes, and the na- ond-order accommodation curve, Figure 12. subaerial exposure on the shallow platform. ture of sequence boundaries are empha- However, the basin/slope region does not In other words, the majority of the flat- sized more than variations in sequence record a similar magnitude or rate of ac- topped shallow platform was exposed dur- thickness to estimate long-term accommo- commodation gain. In fact, rock types and ing many high-frequency sea-level falls/low- dation changes. Similar to high-frequency facies trends of the basin/slope transgressive stands, thus was unable to provide sediment cycle stacking patterns, sequences capped by systems tract 3 and early highstand systems to slope and basin regions; contributions highstand systems-tract peritidal facies re- tract 3 are very similar to transgressive sys- from pelagic calcareous microfossils is not flect full aggradation of the platform to tems tract 2 and highstand systems tract 2, considered because they did not evolve until long-term sea level, thus they provide a despite the major morphologic and appar- the Mesozoic. The lack of evidence of non- more accurate estimate of long-term ac- ent accommodation differences between the deposition or slow deposition in basinal de- commodation changes than do sequences equivalent shallow-platform regions. The posits implies that some of the deposits were dominated by deep subtidal facies. The difference between the apparent magnitude derived locally from the continuously sub- comparison between basin/slope sequences or rate of third-order accommodation gain (dominated by deep subtidal facies) and along the shallow platform and basin/slope merged slope and/or were derived from shallow platform sequences (dominated by during transgressive systems tract 3/maxi- along-slope sediment sources via geostro- peritidal and shallow subtidal facies) pro- mum flooding zone 3 indicates that the ac- phic or contour currents. Relatively high ac- vides a means of evaluating the relative con- commodation gain was not solely the result cumulation rates for basinal facies is indi- tributions of third-order eustasy, subsid- of third-order eustatic sea-level rise. The cated by the fact that sequences 2, 3, and 4 ence, and sediment supply to second-order greater apparent magnitude or rate gain are thicker along the basin/slope than along accommodation trends. along the shallow platform suggests (1) dif- the shallow platform. If sedimentation rates The following discussion of second-order ferential subsidence rates existed between were significantly slower along the basin/ accommodation changes is based on inter- the shallow platform and platform margin, slope, in part resulting from slow deposition pretations of comparisons between shallow (2) apparent deepening was caused by sed- or nondeposition during high-frequency ex- platform and basin/slope sequence stacking imentation rates along the shallow platform posure events, then basinward-thinning ge- patterns. During sequence 1, the evolution lagging behind platform-margin rates, (3) ometries or basin starvation would be ob- from a homoclinal ramp (transgressive sys- the shallow platform was faulted and down- served (e.g., Upper Permian Capitan–Bell tems tract 1) to a distally steepened ramp dropped with respect to the platform mar- Canyon complex of west Texas; Garber et (highstand systems tract 1) reflects an in- gin, and/or (4) basin/slope environments al., 1989). crease in second-order accommodation were insensitive recorders of third-order ac- Rapid shallowing during highstand sys- space as the former shallow-ramp region ag- commodation changes. Given the regional tems tract 3, development of a flat-topped graded to form a nearly flat-topped mor- extent of the intraplatform basin (Ͼ600 ϫ platform, and the concurrent seaward pro- phology (keep-up sedimentation). Slope 100 km; Fig. 1), the ability to correlate se- gradation of the platform margin over the steepening during sequence 1 is interpreted quence 3 systems tracts across the platform, former basin/slope (catch-up style deposi- to reflect differential sedimentation rates and the lack of evidence of Middle Devo- tion) signal a decrease in the rate of second- along the shallow platform (relatively rapid) nian tectonism in Nevada, it is unlikely that order accommodation gain (Fig. 12). Again, versus the deeper platform region (relatively fault-induced subsidence generated the in- the formation of a sequence boundary zone slow). Increasing second-order accommoda- traplatform basin. Presently, it is not clear indicates that relative third-order sea-level

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fall rates were less than tectonic subsidence rates across the study area. A decrease in the rate of second-order accommodation gain during sequence 4 development (Fig. 12) is reflected by the decrease in sequence thickness relative to underlying sequences, development of a thin transgressive systems tract and maximum flooding zone (Ͻ5 m thick) along the basin, and seaward progradation of the platform margin. The rate decrease was not great enough, however, to generate a type 1 sequence-bounding unconformity. Although not described in this study, the extent and apparent magnitude of deepening recorded in the overlying sequence along the basin/ slope and shallow platform (Fox Mountain member of Simonson Dolomite) indicate a significant increase in the rate of second- order accommodation gain related to the Taghanic onlap (Fig. 12; Johnson, 1970). Except for incipient drowning during transgressive systems tract 3 and maximum flooding zone 3, sediment accumulation rates along the Middle Devonian shallow ramps/platforms kept pace with fifth- through second- order accommodation gains, thus generating aggradational to slightly progradational sequence-scale stacking patterns or keep- up style deposition. These sequence-scale stacking patterns and the development of sequence boundary zones rather than sequence-bounding unconformities reflect the effects of second-order accommodation gains related to the lower part of the Kaskaskia sequence of Sloss (1963) (Fig. 12). These long-term, keep-up style sedimentation patterns are similar to those observed in many Cambrian–Ordovician and Mesozoic rimmed- shelf systems (e.g., Read, 1989; examples in Simo et al., 1993). However, the Middle De- vonian deposits lack features that character- ize rimmed shelves such as platform-rim- ming reef complexes or grainstone shoals, and gradients seaward of the platform margins are not steep enough to generate thick accumulations of coarse-sediment gravity- Figure 10. (A) Cross section of sequence 4. Symbols as in Figure 3. Note that sequence flow deposits. The aggraded nature of the 4 is relatively thin, deepening during transgressive systems tract 1 is minor, and facies Middle Devonian shallow ramp/platform along the shallow platform are dominated by peritidal deposits. (B) Partial stratigraphic deposits suggests that fourth- to fifth-order columns illustrating details utilized to interpret systems tracts and boundaries. Symbols sea-level oscillations were of low magnitude as in Figure 4B. Scale in meters. (Ͻ10 m), which allowed sediment accumulation rates to keep pace with the combined high-frequency and third-order sea-level minor continental ice sheet development dational over meters to tens of meters, in- rise rates. These low-magnitude, fourth- to (Fischer, 1981; Frakes et al., 1992; Wright, dicating tectonic subsidence rates were fifth-order sea-level oscillations support in- 1992) (see discussion in Elrick, 1995). greater than sea-level fall rates, and fourth- terpretations for Middle Devonian green- Low-magnitude or slow rise/fall rates of to fifth-order cycles are absent in intraplat- house climate conditions that are character- third-order sea-level oscillations are also form basin, slope, and basinal facies. If ized by globally warm temperatures, high suggested from the Middle Devonian depos- third-order magnitudes or rise/fall rates mean ocean temperature, and as a result, its because each sequence boundary is gra- were greater, then during third-order low-

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noncyclic deep subtidal facies, or distinct deeper subtidal units within a single cycle. Highstand systems tracts along the basin/ slope are composed of fine-grained hemipelagic deposits overlain by distal through proximal turbidites, which may be capped by platform-margin grainstones. Along the shallow platform, highstand systems tracts are well defined by upward-shallowing successions of cyclic shallow subtidal through peritidal facies. Sequence boundary zones (2–16 m thick) along the shallow platform are composed of stacked, exposure-capped peritidal and subtidal cycles that exhibit upsection increases in the proportion of tidal-flat subfacies and concurrent increases in the intensity of cycle-capping subaerial exposure features. These cycle-capping exposure features indicate that the shallow platform was affected by multiple episodes of subaerial exposure rather than a single, long-lived exposure event. Sequence boundary zones along the basin/slope (6–20 m thick) are composed of upward-shallowing successions of proximal Figure 11. Schematic depositional profiles of platform morphology during sequence 4. turbidites or platform-margin grainstone -section TM. (A) During deposition of late transgressive shoal deposits, which are immediately over ؍ Symbols as in Figure 7; TM systems tract 4/maximum flooding zone 4, a gentle dipping ramp with a distally steepened lain by basinal facies. The lack of exposure edge developed between sections NA and TM. Landward of section TM, a wide region of features in these sequence boundary zones shallow subtidal and peritidal facies was deposited, indicating the platform was relatively indicates that third- through fifth-order sea- flat topped. (B) Seaward progradation of cyclic peritidal facies and peloid-ooid grainstones level fall rates were less than tectonic sub- during late highstand systems tract 3 resulted in the generation of a flat-topped platform sidence rates along the basin/slope. across the entire study area. Because of late highstand systems tract 3 progradation, the (3) High-resolution conodont and bra- location of the slope break and the gradient along the slope cannot be observed within the chiopod biostratigraphy permits correlation study area. across contrasting facies belts of the basin/ slope and platform margin, while stratal stacking patterns and exposure features en- stands, superposed high-frequency oscilla- regressive sequences (Ϸ1.5–2.5 m.y. dura- able correlation across the rest of the tions would have affected sedimentation tions) are identified based on shallowing shallow platform where biostratigraphically patterns in more of the deeper water envi- and deepening patterns in regional facies, diagnostic fossils are absent. The four se- ronments. For example, along the intraplat- intensity and stratigraphic distribution of quences are biostratigraphically correlated form basin during high-frequency sea-level subaerial exposure features, and stacking with previously dated, uppermost Lower– fall and lowstand, peritidal facies would patterns of fourth- to fifth-order, upward- Middle Devonian transgressive-regressive have prograded over intrashelf basin facies shallowing cycles. sequences in the western and eastern to generate meter-scale peritidal cycles, (2) Depositional sequences are 30–180 m United States, western Canada, and Eu- while along the slope, platform-margin thick and can be correlated across the entire rope, indicating they are eustatic in origin. grainstones would have prograded over ba- platform-to-basin transition. Transgressive (4) Subsequence-scale correlations across sin and slope facies to generate meter-scale, systems tracts along the basin/slope are the study area indicate that platform mor- grainstone-capped cycles. characterized by upward-deepening succes- phologies alternated between distally steep- sions of proximal through distal turbidites ened ramps during transgressive systems CONCLUSIONS overlain by fine-grained hemipelagic depos- tract development and flat-topped plat- its (sequences 2, 3, and 4). Transgressive sys- forms during highstand systems tract de- (1) Uppermost Lower–Middle Devonian tems tracts along the shallow platform are velopment. Except for incipient drowning carbonates (270–400 m thick) of the eastern composed of successions of peritidal cycles during transgressive systems tract 3 and Great Basin were deposited along a west- overlain by subtidal cycles, successions of maximum flooding zone 3, the sequences ward-thickening, carbonate platform. Six re- thicker-than-average peritidal cycles, or by stack in an aggradational to slightly pro- gional facies representing peritidal, shallow noncyclic deep subtidal facies. Along the gradational pattern (keep-up style sedi- subtidal, stromatoporoid buildup, deep sub- shallow platform, maximum flooding zones mentation) and are bounded by sequence tidal, slope, and basinal environments are are defined by succession of peritidal cycles boundary zones rather than unconformities, recognized. Four third-order, transgressive- dominated by subtidal units, intervals of suggesting that greenhouse climate modes

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Figure 12. Devonian third-order, eustatic sea-level curve (or more correctly termed, a relative paleobathymetric curve) modified from Johnson et al. (1985). The second-order accommodation curve represents the envelope of third-order highstand positions and reflects the increase in accommodation associated with the lower part of the Kaskaskia sequence of Sloss (1963). Note that incipient drowning associated with sequence 3 is interpreted as the second-order maximum flooding zone. Third-order segments labeled Ia–IIf are previously recognized deepening events (T-R cycles of Johnson et al., 1985, in press) in the western, midwestern, and eastern United States, western Canada, and Europe. Biostratigraphic and sequence stratigraphic correlations recognized in this study indicate that sequences 1, 2, 3, and 4 correlate with T-R cycles Ic, Ib, Ie, and If, respectively.

and second-order accommodation gains re- ‘‘Jess’’ Johnson for our invaluable discus- Burchfiel, B. C., and Royden, L. H., 1991, Antler orogeny: A Med- iterranean-type orogeny: Geology, v. 19, p. 66–69. lated to the lower part of the Kaskaskia se- sions about the Devonian stratigraphy/bio- Cisne, J. L., 1986, Earthquakes recorded stratigraphically on carbonate platforms: Nature, v. 323, p. 320–322. quence controlled sequence-scale stacking stratigraphy of the Great Basin. The manu- Cook, H. E., and Taylor, M. E., 1977, Comparison of continental patterns. script benefited from reviews by Todd slope and shelf environments in the Upper Cambrian and Lowest Ordovician of Nevada, in Enos, P., and Cook, H. E., LaMaskin, Katie Giles, Brian Pratt, Carl eds., Deep-water carbonate environments: Society of Eco- nomic Paleontologists and Mineralogists Special Publica- ACKNOWLEDGMENTS Drummond, Charles Kahle, Jim Markello, tion 25, p. 51–81. Steve Reid, and Steve Dorobek. Cowan, C. A., and James, N. P., 1993, The interactions of sea-level change, terrigenous-sediment influx, and carbonate produc- Funding for this project was provided by tivity as controls on Upper Cambrian grand cycles of west- the American Chemical Society (PRF ern Newfoundland, Canada: Geological Society of America REFERENCES CITED Bulletin, v. 105, p. 1576–1590. #25593-G8) and a Research Allocations Demicco, R. V., 1983, Wavy and lenticular-bedded carbonate rib- Ahr, W. M., 1973, The carbonate ramp—An alternative to the bon rocks of the Upper Cambrian Conococheague lime- Committee grant from the University of shelf model: Gulf Coast Association of Geological Societies stone, central Appalachians: Journal of Sedimentary Petrol- New Mexico. Field assistance was gratefully Transactions, v. 23, p. 221–225. ogy, v. 53, p. 1121–1132. Bond, G. C., and Kominz, M. A., 1984, Construction of tectonic Dobbs, S. W., Carpenter, J. A., and Carpenter, D. G., 1993, Struc- received from Tom Oesleby, Chris Androni- subsidence curves for the early Paleozoic miogeocline, tural analysis from the Roberts Mountains to the Diamond southern Canadian Rocky Mountains: Implications for sub- Mountains, Nevada: Estimates on the magnitude of con- cus, Bob Goldhammer, Gabriella Savarese, sidence mechanism, age of breakup and crustal thinning: traction and extension, in Gillespie, C. W., ed., Structural Sue Rotto, Katerina Petronotis, and Mark Geological Society of America Bulletin, v. 95, p. 155–173. and stratigraphic relationships of Devonian reservoir rocks, Burchette, T. P., and Wright, V. P., 1992, Carbonate ramp dep- east-central Nevada: Reno, Nevada Petroleum Society, 1993 Boslough. My gratitude goes to the late J. G. ositional systems: Sedimentary Geology, v. 79, p. 3–57. Field Conference Guidebook, p. 51–57.

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416 Geological Society of America Bulletin, April 1996

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